Neutronography is a technique for non-destructive testing analogous in its principle to radiography using X-rays. A conditioned beam of neutrons from a source is used to supply an image by transparency of the object being examined.
A conventional neutronography installation is represented schematically in FIG. 1 which shows: a source of neutrons SN placed in a moderator medium, designed to reduce the energy of the neutrons that are produced. The entry window of a collimator C, which conditions the beam, is disposed at a calculated distance from this source. In this figure, a collimator with a divergent shape is shown. The collimator is equipped with a filter F made of a material designed to act as a barrier to the gamma radiation that can accompany the neutron transmission. The collimator comprises internal walls absorbing neutrons and a filling gas transparent to neutrons. At the exit of the collimator, the neutrons interact with the object under examination Ob, are transmitted through the latter and are absorbed by a detector Dect for forming the image. The use of a moderator element is indeed necessary given that, in general, the neutron sources emit a spectrum of fast neutrons which is very rarely suitable for imaging. The neutrons then need to be slowed down in a medium referred to as moderator surrounding the fast source. This method unfortunately leads to considerable losses in intensity because the moderator is not totally transparent to the neutrons and spurious captures are always taking place within it. In the moderator, the neutrons are slowed by successive collisions accompanied by transfers of energy to the atoms of the medium.
The object thus analyzed and traversed by a neutron flux generates an image that is specific to it. Indeed, the interaction of the neutrons within the material is characterized by scattering and absorption phenomena. The neutrons have, in particular, the capacity to detect atoms of hydrogen in media through metal structures, and the analysis can be undertaken by the spatial and temporal attenuation of the beam of neutrons.
Research nuclear reactors, producers of very high neutron fluxes, are the installations that are best placed for producing neutronographies of very high quality. In fact, they occupy a very important place amongst the installations dedicated to production non-destructive testing.
Since neutrons are indirectly ionizing particles, their direct detection is quite difficult. In order to overcome this handicap, in the detector, a material with a very large cross-section (probability of interaction) for neutron capture is used in order to obtain a high efficiency, this capture being accompanied by a secondary transmission of ionizing particles that can excite conventional detectors: photographic films, scintillators (material which emits light following the interaction with ionizing radiation (photon or charged particle)). Indeed, the light from scintillation is produced not only by absorption but also by other types of interactions with ionizing radiation such as scattering, for example.
In the nuclear field, two types of neutronography installations exist: neutronography in the reactor fuel pond, in which the imaging system is installed as close as possible to the core of the reactor right inside the pond (the system is immersed, which is for example the case of the OSIRIS neutronography system in France or the HFR at Petten in the Netherlands) and neutronography outside the reactor, in which the beam of neutrons is extracted from the reactor in order to form a beam exiting from the reactor fuel pond (the ORPHEE neutronography system for example).
OSIRIS is an experimental reactor with a thermal power of 70 megawatts. This is a light-water pond type of reactor with an open core whose main goal is to carry out testing and to irradiate fuel elements and structural materials for high power nuclear electrical plants with a high flux of neutrons and also to produce radioelements.
In the case of the examination of irradiated fuel elements, the appropriate neutronography system is immersed neutronography, for obvious reasons associated with the irradiating nature of the object under examination.
The severe constraint associated with this type of neutronographic imaging immersed in a reactor fuel pond of highly-irradiating objects is linked to the type of detector used to form the neutronographic image. Owing to the radioactive environment, the system must be insensitive to gamma radiation so as only to conserve the useful signal (interactions with the thermal neutrons). Currently, the device used in the framework of OSIRIS for performing the neutronographic analysis and installed on the bed of the pond is composed of three main parts:                a pyramidal collimator whose apex is slightly truncated where an aluminum alloy plate is placed forming the entry field for the neutrons;        at the rear of the collimator is located the chamber which receives the object to be examined;        the support situated at the rear of the chamber receives a metal cassette containing a converter capable of converting a neutron flux into β radiation. The cassette provides a function of isolation vessel with respect to the neutron converter designed to be immersed during the irradiation phase.        
The whole assembly is mobile and moves forward or backward toward or away from the core of the reactor. The technique is referred to as a transfer technique because the radiographic image is obtained following two consecutive sequences:                the irradiation of the converter;        the exposure of a photographic film after transferring the cassette outside of the pond in order to image the β radiation activity of the converter.        
This method allowing only the neutron signal to be conserved is thus founded on a production of the image in two steps, after a transfer of a part of the system outside of the reactor fuel pond: an activatable converter is exposed to the neutron flux downstream of the object being inspected, then this converter is transferred outside of the pond in order to image its activity (beta activity), which supplies an image in transmission of the neutron absorption of the object being inspected. It offers the advantages of being able to test an object directly in the neighborhood of the core without removing it from the pond and of being able to image highly radioactive objects because the photographic film is never in the neighborhood of the latter. Thus, the irradiated fuel rods in an experimental device OSIRIS can be subjected to a neutronography before and after irradiation which allows the modifications of the state of the fuel and the effect of the irradiation to be seen.
Nevertheless, the systems enabling the production of neutronographic images under these conditions require the employment of a system that is transferable outside of the reactor fuel pond of the irradiation cassette type composed of aluminum alloy or other (activatable) metal material. The activation of this cassette imposes severe constraints on the measurement process (feedback from OSIRIS experiment): handling times, dosimetry for the operator.
Furthermore, the methods for neutronography by transfer currently employed impose the application of a primary converter for converting neutrons into remnant beta radiation, on a second system allowing the image to be produced (radiographic film or radio-luminescent storage screen), involving the necessity of bringing the two elements into the most perfect contact possible in order to optimize the resolution of the image (use of a vacuum box, etc.) and many handling operations by the operator.